Difference between revisions of "Linux-AWK/C2/Variables-and-Operators-in-awk/English-timed"

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Latest revision as of 20:50, 16 July 2019

Time
Narration
00:01 Hello and welcome to this spoken tutorial on Variables and operators in awk command.
00:07 In this tutorial, we will learn about-

User defined variables,

00:12 Operators,

BEGIN and END statements.

00:17 We will do this through some examples.
00:20 To record this tutorial, I am using Ubuntu Linux 16.04
00:26 To practice this tutorial, you should have gone through the earlier Linux tutorials on this website.
00:33 You should be familiar with basic operators used in general programming languages like C or C++.
00:41 If not, then please go through the corresponding tutorials on our website.
00:47 awk combines the power of a filter and a programming language.
00:52 So, it supports variables, constants, operators etc.
00:58 Let’s see what a variable in awk is.
01:02 A variable is an identifier that references a value.
01:07 Awk supports both user-defined variables and built-in variables.
01:12 We will learn about user-defined variables in this tutorial.
01:17 For user-defined variables, variable declaration is not required.
01:22 Variables do not have to be initialized explicitly.
01:26 Awk automatically initializes them to zero or null string.
01:32 A variable must begin with a letter and continue with letters, digits and underscores.

Variables are case-sensitive.

01:43 So, Salary with capital “S” and salary with small “s” are two different variables.
01:50 Let us look at some examples now.
01:53 Open the terminal by pressing CTRL + ALT and T keys.
01:58 On the terminal, type- awk space opening single quote opening curly brace small x equal to 1 semicolon capital X equal to within double quotes capital A semicolon small a equal to within double quotes awk semicolon small b equal to within double quotes tutorial.

Press Enter.

02:25 Type: print x, press Enter.
02:29 print capital X, press Enter.
02:34 print a, press Enter.
02:37 print b, press Enter.
02:40 print a space b, press Enter.
02:44 print small x space b, press Enter.
02:49 print small x plus capital X closing curly brace closing single quote and press Enter.
02:57 Since we have not given a filename, awk would need some input from standard input.
03:03 And hence, we can type any letter, say a and then press Enter.
03:10 This example shows a couple of things.

Variables can be initialized with a number.

03:18 It can also be initialized with value as a single character or a string.
03:23 If the value is a character or a string, variable is initialized with value within double quotes.
03:31 We can see the values of the variables.
03:35 Observe that small x and capital X are treated as different variables.
03:41 This proves that variables are case sensitive.
03:45 Also, it shows how two strings can be concatenated.
03:50 Here variables small a and small b are concatenated.
03:55 So, string concatenation operator is simply a space.
04:00 Similarly, when we concatenate small x which is a number and string b, x is auto-converted into string.

And the concatenated output becomes 1tutorial.

04:13 Why does the auto-conversion to string happen?
04:16 That's because awk finds a string concatenation operator space here, between x and b.
04:25 Now, look at the output of small x plus capital X.

Here, we have the arithmetic operator plus.

04:33 So, X is auto-converted to numeric zero.

And the addition output becomes numeric 1.

04:42 Until now, we have seen a couple of operators.

Let’s look at what other operators we can use.

04:49 A variety of operators can be used in expressions.
04:53 Please pause the video here and take a look at all the operators mentioned here.
04:58 I assume you are familiar with these basic operators.
05:02 If not, then kindly visit our website for tutorials on operators in C and C++ series.
05:09 I am not going to discuss the working of all these operators in detail.
05:14 Only exception is the string matching operator which may be new to you.

Let us understand this with an example.

05:23 A file named awkdemo.txt has been provided in the Code files link.

Please download it on your computer.

05:31 Switch to the terminal.

Let us end the previous process by pressing Ctrl and D keys.

05:38 Let me clear the terminal.
05:41 Now, go to the folder in which you have saved the awkdemo.txt file using the cd command.
05:48 Let us have a look at this file now.
05:52 Let's say we want to find the students who have passed but have marks less than 80.
05:58 In this case, we need to compare two different fields.
06:02 For such situations, we can use awk's relational operators.
06:07 These operators can compare both strings and numbers.
06:12 So, on the terminal, type:

awk space hyphen capital F within double quotes vertical bar space within single quotes dollar 5 equal to equal to within double quotes Pass space ampersand ampersand space dollar 4 less than 80 space within curly braces print space plus plus x comma dollar 2 comma dollar 4 comma dollar 5 space awkdemo.txt and press Enter.

06:54 This command shows a number of things.

One, we compare a string with the fifth field.

07:01 Second, we only compare the fourth field with a number.
07:06 Third, we see that we can join two or more comparisons using ampersand operator.
07:13 Instead of specific numbers or strings, we can also compare regular expressions.
07:19 As we have seen in the slide, we have the tilde and the exclamation tilde operators for this purpose.
07:27 Now suppose, we want to find computer science students who have passed.
07:32 Since computers can have both a small and capital C, we would use a regular expression.
07:40 We would type:

awk space hyphen capital F within double quotes pipe symbol space within single quote dollar 5 equal to equal to within double quotes Pass ampersand ampersand space dollar 3 tilde slash within square brackets small c capital C computers slash space within curly braces print space plus plus small x comma dollar 2 comma dollar 3 comma dollar 5 space awkdemo.txt and press Enter.

08:24 If we want to negate the comparison, we can do so using the exclamation tilde operator.
08:30 Say, now we want a list of all non-computer students who have passed.
08:35 Use the up-arrow to get the previous command.
08:39 Next to dollar 3 add exclamation symbol and press Enter.
08:47 Next, let's count the number of blank lines in the same file.
08:52 Open the file and check how many blank lines are there.

So, it has 3 blank lines.

09:00 Now to count the number of empty lines using awk, type:

awk space within single quote within front slash caret symbol dollar space within curly braces x equal to x plus 1 semicolon space print x space awkdemo.txt

Press Enter.

09:26 We got 3 as our final answer.
09:30 The caret sign signifies start of a line while dollar signifies the end of a line.
09:37 Hence, an empty line would be matched by the regular expression caret-dollar.
09:43 Note, we have not initialized the value of x.

Awk has initialized x to the initial value zero.

09:51 This command gives us the running count of blank lines.

This is because, every time a blank line is found, x would be incremented and then printed.

10:02 In our last command, we have seen running count of blank lines.

But, say we only want to print the total number of blank lines.

10:12 Then we need to print x only once, after the entire file has been traversed.
10:19 We may also want to give a heading saying what the output means.
10:25 For such requirements, awk provides the BEGIN and the END sections.
10:30 The BEGIN section contains procedures for pre-processing.
10:34 This section is executed before the main input loop is executed.
10:40 The END section can contain procedures for post-processing.
10:45 This section is executed after the main input loop has terminated.

The BEGIN and END procedures are optional.

10:55 Let's learn how to do this.

In the terminal, type:

awk space opening single quote BEGIN in caps within curly brace print space within double quotes The number of empty lines in awkdemo are press Enter.

11:14 within front slash caret symbol dollar symbol space within curly braces x equal to x plus 1

press Enter.

11:26 end space within curly braces print space x close single quote space awkdemo.txt and press Enter.
11:39 See, we did not get the desired output!

We should get the output as 3 because we have 3 blank lines in the file.

11:48 What do you think happened?

Actually, we should have written end as upper case END.

11:54 So, let us modify the command.
11:57 Press the up-arrow key to get the previous executed command on the terminal.
12:03 Now, modify lower case end to upper case END.

And press Enter.

12:11 Now the total number of empty lines is displayed in the output.
12:16 Next, let's find the average salary of all the students that we found in the awkdemo.txt file.
12:24 To get that, type the command as shown in the terminal and press Enter.

And, we get the desired output.

12:35 This brings us to the end of this tutorial. Let us summarize.
12:40 In this tutorial, we learnt about:

User defined variables in awk,

12:45 Operators,

BEGIN and END statements.

12:49 As an assignment, print every line where the value of the last field is more than 5000.

And, the student belongs to Electrical department.

13:00 Print the average marks of all the students with the heading “Average marks” in the output.
13:07 The video at the following link summarises the Spoken Tutorial project.

Please download and watch it.

13:14 The Spoken Tutorial Project team conducts workshops using spoken tutorials

and gives certificates on passing online test.

13:23 For more details, please write to us.
13:27 Do you have questions in THIS Spoken Tutorial?

Please visit this site.

13:32 Choose the minute and second where you have the question.

Explain your question briefly.

Someone from our team will answer them.

13:42 The Spoken Tutorial forum is for specific questions on this tutorial.
13:47 Please do not post unrelated and general questions on them.
13:51 This will help reduce the clutter.

With less clutter, we can use these discussion as instructional material.

13:59 Spoken Tutorial Project is funded by NMEICT, MHRD, Government of India.

More information on this mission is available at this link.

14:10 The script has been contributed by Antara and this is Praveen from IIT Bombay, signing off.

Thank you for joining.

Contributors and Content Editors

PoojaMoolya, Sandhya.np14